In recent years---even in the U.S., reversal rather than continued progress has typified efforts to control biting flies and the numerous diseases they transmit. Future progress will depend on a better basic understanding of vectors and how they interact with vertebrate populations. This interface seems to represent the weakest link in many disease cycles. This proposal primarily deals with how certain vectors succeed in feeding on and infecting vertebrates that are normally unavailable to other haematophagous Diptera. We will test rodent blood meals (by steroid hormone binding assay) for fetoprotein (only present in suckling young) to determine if pre-weaned animals are exploited as hosts by certain mosquitoes. A similar test may be developed for aging the avian hosts of Cs melanura and Culex spp. to determine when and how often vectors feed on non-immune nestlings. Observations in a transparent burrow system will assess whether rodent-feeding mosquitoes normally penetrate burrows in their quest for blood. Our evidence that rodents can be orally infected with malaria by eating infective mosquitoes will be expanded to include viruses and will be evaluated in terms of epidemiological significance. Our working hypothesis that parasitemic hosts are more vulnerable to vector feeding will be further explored using bird malaria/Culex pipiens and rodent malaria/Anopheles stephensi systems. A variety of serological and observational methods will be used to uncover the key factor(s) which maintain known mosquito-rodent and mosquito-bird associations involving several human viruses (VEE, LAC, EEE, SLE). A large wind tunnel will be employed to study the visual responses of mosquitoes and to determine if individual females respond to visual and olfactory cues in a repetitive manner. Finally, recent insight concerning black fly resting behavior will be exploited to collect significant numbers of naturally fed flies for serological analysis of host-blood types.